Currently, the enhancement of uplink performance in wideband code division multiple access (WCDMA) of universal mobile telecommunication system (UMTS) is under development. Especially, high speed uplink packet access (HSUPA) is under development and standardization. In combination with this enhancement of uplink performance of mobile WCDMA-enabled terminals (also denoted as user equipment UE, e.g. a mobile UMTS-enabled terminal) communicating to base stations (also denoted as node B) fast hybrid ARQ (automatic response request) is to be employed for increasing the uplink data communication speed as well as the uplink data communication capacity which effects both the individual uplink data communication speed and capacity of a mobile UMTS terminals and the total uplink data communication speed and capacity within the coverage of a base station.
In wideband code division multiple access (WCDMA) several individual in principle independent information parts are combined using channelization procedures and scrambling procedures, respectively, to communicate these information parts at the same time via one physical channel (i.e. within one frequency band). Channelization procedures and scrambling procedures are operated in such a manner, respectively, that the individual information parts are separately obtainable at the receiving side by inverse channelization procedures and inverse scrambling procedures, respectively. In general, a channelization procedure serves to combine (or code) several individual information parts of for example one mobile WCDMA-enabled terminal whereas a scrambling procedure serves to allow a differentiation of signals generated and transmitted by for example different mobile WCDMA-enabled terminals.
From the energy point of view the all signals referring to the aforementioned information parts are superimposed in the same frequency band at the same time and consequently each signal power adds to the total signal power transmitted within the same frequency band. But unfortunately the total signal power transmitted and the interference and noise power are associated with each other. Therefore and due to other reasons power control is essential in code division multiple access (CDMA) systems and hence also in wideband code division multiple access (WCDMA) systems.
A first main concern addressed by power control is denoted as the near-far problem. In case of several transmitting CDMA-enabled terminals being spaced at different intervals from a receiving base station the nearby transmitting CDMA-enabled terminal may overpower remote transmitting CDMA-enabled terminals since the signals of all the transmitting CDMA-enabled terminals are superimposed in the same frequency band at the same time. A second main concern affects the Raleigh fading of physical transmission channels which may be a result of multi-path propagation. Two main techniques are involved in the power control for transmission signals in wideband code division multiple access (WCDMA) systems, the open-loop power control and the closed-loop power control.
Open-loop power control allows a terminal for estimating required-transmission power based on the signal power received from the base station and information about the original transmit power of the base station. This results to a rough estimation of the required transmission power.
Closed-loop power control allows a receiver (either a terminal or a base station) to command a transmitter (either a base station or a terminal) to adjust the transmission power. Commands for adjusting the transmission power are based on signal-to-interference ratio (SIR) measurements carried out on the receiving side. The closed-loop power control is aimed at adjusting the transmission power such that the measured SIR value is as close as possible to a pre-determined target SIR value. These commands are transmitted on physical control channels associated with physical data channels in each time slot corresponding to a data packet (in case of 15 time slots per each 10 ms 1500 times per second).
Additionally, a quality of service based power control technique is also employed in order to maintain a required or desired level of the quality of service (for example an effective data rate for a real-time application). In general, a maintaining of a required or desired level of the quality of service is reached by maintaining a SIR value at a corresponding level at the receiving side. However, the target SIR value is a function of the quality of service, for example in case the quality of service is expressed in terms of a frame error rate (FER) on the air interface the target SIR value is a function of the FER. A service such as a speech service or a data service of low or high data rate determines the acceptable FER and therefore the target SIR. The outer-loop power control allows for varying the target SIR in accordance with a quality of service requirement which indirectly concerns the transmission power by affecting the closed-loop power control.
Besides the above presented power control further quality of service enhancing features are applied in cellular communication systems and hence particularly in wideband code division multiple access (WCDMA) systems. Adaptive modulation and coding schemes (AMC) provide the flexibility to adapt individually the modulation and coding scheme of data to average channel conditions for each transmitter within a certain transmission time frame. Adaptive modulation and coding (AMC) is based on an explicit carrier-to-interference ratio (C/I) measurement or related measurements. Traditionally in CDMA systems (and hence also in WCDMA systems) fast power control is preferably employed to adapt data transmission to varying link (channel) conditions.
Hybrid automatic repeat request (H-ARQ) is an implicit link adaptation technique. In H-ARQ, link layer acknowledgements are used for re-transmission decisions. There are many schemes for implementing H-ARQ-chase combining, rate compatible punctured turbo codes and incremental redundancy (IR). Incremental redundancy (IR) or H-ARQ type II is another implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ type-III also belongs to the class of incremental redundancy (IR) ARQ schemes. However, with H-ARQ type III, each retransmission is self-decodable which is not the case with H-ARQ-type II. Chase combining (also called H-ARQ type III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received signal-to-noise ratio (SNR). Diversity (time) gain is thus obtained. In the H-ARQ type-III with multiple redundancy version different puncture bits are used in each retransmission.
Currently in H-ARQ leaving aside aforementioned power control adaptation, the re-transmission power of data packets is equal to the transmission power of the original (corrupt) data packet. For example, in case of an implementation of H-ARQ type I with chase combining, the original data packet and the (first) re-transmitted data packet are combined at the receiving side under consideration of their respective received SNR values (weighted by their received SNR values). That is from energy point of view, the energy of the original data packet and the energy of the (first) re-transmitted data packet are added (combined) weighted by their SNR values. The resulting combined data packet has a combined SNR value corresponding to the SNR values. In case the combined SNR value is lower than a decode threshold value decoding of the combined data packet is not possible and a new re-transmission for combining is required. In the other case the combined data packet having a combined SNR value being equal or exceeding the decode threshold value is decodable. The exceeding of the combined SNR value above the required decode threshold value is not necessary for decoding and limits the performance of data communication.